OSLO (Reuters) -- The world will have to axe greenhouse gas emissions by 80 percent by 2050, more deeply than planned, to have an even chance of curbing global warming in line with European Union goals, researchers said on Thursday.

Even tough long-term curbs foreseen by the EU or California fall short of reductions needed to avert a 2 degrees Celsius (3.6 Fahrenheit) temperature rise over pre-industrial times, seen by the EU as a threshold for "dangerous change", they said.

"Any delay in implementing emissions reductions will make a 2 degree target practically unreachable," he and colleague Steffen Kallbekken wrote of findings to be published in the journal Climatic Change.

The EU reckons that there would be dangerous disruptions to the climate such as ever more droughts, heat waves, floods and rising seas beyond a 2 C ceiling. Temperatures already rose by about 0.7 Celsius in the 20th century.

An 80 percent global cut would mean rich nations, responsible for most heat-trapping emissions from fossil fuels burnt by power plants, factories and cars, would have to axe emissions by about 95 percent below 2000 levels by 2050.

Developing countries such as China, India, Brazil and Indonesia, where emissions are rising sharply in line with energy use to help lift millions from poverty, would have to take on less swinging reductions, they said.

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"Even the most ambitious proposals for emissions cuts in 2050, such as the UK draft climate bill which sets a cut of 60 percent, or the California target to reduce emissions by 80 percent by 2050, fall short," they said.

A draft report by the U.N. climate panel due for release on May 4 in Bangkok also concludes that a maximum 2 C rise would be hard to achieve. Restraints on emissions consistent with the goal could cost up to 3 percent of world gross domestic product.

And Kalbekken and Rive said that global emissions would have to peak in 2025, with cuts in place by 2010, to achieve an 80 percent cut by mid-century. Any delays would sharply raise costs.

Under the U.N.'s Kyoto Protocol, 35 industrialized nations now have goals of cutting emissions by 5 percent below 1990 by 2008-12. The United States, which says the plan is too costly and wrongly excludes developing states, is the main outsider.

U.N. climate negotiations focused on widening Kyoto beyond 2012 are stalled. Developing nations say they cannot be expected to cap emissions when energy use has been a key to economic growth by rich states since the Industrial Revolution.